1. Field of the Invention
The present invention relates to the manufacture of thin-film components. In particular, the present invention concerns a method for growing a ZnS:Mn phosphor layer for use in inorganic thin-film electroluminescent components, wherein the ZnS:Mn phosphor layer is grown on a substrate by means of the Atomic Layer Epitaxy method. According to the method, volatile (i.e., gaseous or vaporizable) zinc, sulfur and manganese compounds are used as precursors for preparing the layer to be deposited.
2. Description of Related Art
Flat electroluminescent displays are used in applications requiring a wide viewing angle, a wide temperature range and a rugged device structure. The most important electroluminescent phosphor material is manganese-doped zinc sulfide (ZnS:Mn) which is conventionally used in all monochrome electroluminescent displays emitting yellow light. Also polychrome electroluminescent displays emitting red and green are based on the same phosphor material, whereby the red and green colors are obtained by filtration from the emission spectrum of ZnS:Mn.
Thin-film electroluminescent (TFEL) layers of ZnS:Mn have been made in the art by means of different methods: evaporation, sputtering, metalorganic chemical vapor deposition (MOCVD) and atomic layer epitaxy (ALE). Of these methods, evaporation and ALE are used in the commercial manufacture of TFEL displays. In an industrial process, both the economic efficiency of the method and the performance of the electroluminescent ZnS:Mn thin film thus obtained are crucial.
Of gas-phase deposition methods, the most widely used are MOCVD and ALE. In the MOCVD method, the precursors are introduced simultaneously into the reactor. The film growth mechanism is principally based on the pyrolysis of the precursors on the substrate surface, whereby the mass flow rates of the precursors can be adjusted to essentially control the film growth rate. In the ALE method, the precursors are sequentially pulsed into the reactor. Herein, the growth mechanism is not based on pyrolysis, but instead, on exchange reactions on the substrate surface. Thus, a self-limiting growth rate is obtained, that is, the growth rate is independent from the mass flow rates of the precursors. Therefore, it is more difficult in the MOCVD method to achieve good uniformity of the deposited film, which explains why this method has not been used in the large-scale production of electroluminescent ZnS:Mn thin-film components. In contrast, the ALE method uses a clearly different approach, whereby thin films made by means of this process exhibit sufficiently uniform thicknesses and compositions for the commercial manufacture of electroluminescent displays.
Conventionally, the best electroluminescent ZnS:Mn thin films made using the ALE method have been prepared using zinc chloride, manganese chloride and hydrogen sulfide as precursors, whereby the substrate surface temperature has been in the range from 500 to 520.degree. C. (cf. FI Patent Specification No. 86995). The same process is also used in the industrial production of electroluminescent displays. Herein, the substrate surface is alternately subjected to reactions with zinc chloride and hydrogen sulfide, whereby a molecular zinc chloride layer adsorbed on the surface reacts with the hydrogen sulfide forming zinc sulfide [J. Hyvarinen, M. Sonninen and R. Tornqvist:
Journal of Cryst. Growth, Vol. 86 (1988), p. 695].
Up to date, hundreds of thousands of thin-film electroluminescent ZnS:Mn displays have been successfully made using the ALE method using chlorides as precursors for depositing ZnS:Mn. However, this conventional technique involves a few evident disadvantages. An example of these is the asymmetric light emission of the electroluminescent ZnS:Mn structure. This phenomenon is elucidated in the appended FIG. 1a. As is evident from the diagram, one polarity of the pixel drive voltage produces higher light emission than the other polarity. In practice, this sets limitations to the use of frequency modulation in the generation of different gray levels on an electroluminescent display, because visible flicker of the emitted light will occur at low drive frequencies due to the asymmetric light emission.
Chiefly due to the low vapor pressure of manganese chloride, the chloride process presupposes a substrate temperature of at least 500.degree. C. This temperature is already very close to the softening point of soda lime glass. Because a soda lime glass substrate is favored on economical grounds, the process temperature must be kept close to 500.degree. C. (in the range 500-520.degree. C.), although layers of improved performance could be obtained at a higher temperature. Because soda lime glass is subject to softening already in the temperature range conventionally used, this phenomenon gives rise to extra costs in the manufacturing process of EL displays.
For the handling of both zinc and manganese chloride, the ALE deposition equipment must be provided with sublimation sources which are both clumsy to handle and difficult to control. Furthermore, as zinc chloride forming the matrix of the thin-film compound is consumed at a higher rate of the two precursors, zinc chloride is obviously the one causing more problems.